1. Field of the Invention
The present invention relates to an optical device, in particular an optical switching device.
2. Description of the Related Art
As is known, switching devices are essential elements for telecommunications and have the purpose of appropriately routing the light beams transmitted along waveguides.
At present, both opto-mechanical and thermo-optical switching devices are available. Opto-mechanical switching devices have good insertion loss and crosstalk characteristics, but are not reliable on account of the presence of moving parts. Thermo-optical switching devices do not have problems of reliability linked to moving parts, but have poorer characteristics in terms of crosstalk and enable implementation of only a small number of waveguides (eight or sixteen).
In order to overcome the above limitations, different solutions are being studied that are based on semiconductor technology, in which waveguides, formed in a silicon substrate, comprise micromirrors formed at the switching points. Also undergoing study are solutions in which the waveguides are formed in a glass plate bonded to a silicon substrate (the latter technology being generally referred to as xe2x80x9cPlanar Lightwave Circuit Technology on Silicon Substratexe2x80x9d).
Aspects of the present invention regard a switching device of the latter type, including a glass plate having a plurality of input/output openings between which waveguides that intersect one another extend (switching matrix). In the crossover points, the waveguides have cavities filled with a liquid having the same refractive index as the waveguide.
The light beams that traverse the waveguides are thus not deviated by the liquid present in the crossover points and exit the switching device in the original direction with a given insertion loss caused by the cavities.
Underneath each cavity, inside the substrate on which the glass plate is bonded there is present a resistor, which, when traversed by current, generates heat by the Joule effect and causes a change of state of the liquid present in the cavity. In practice, activation of the resistors brings about formation of bubbles in the crossover points. Given that the bubbles have a refractive index different from that of the original waveguide, the light beam that passes through a crossover point in which an air bubble is present is deviated with respect to the original direction, towards a different waveguide.
Switching of the light beam is associated to a given crosstalk (for example, in the region of 50 dB) and to pre-set insertion losses (for example, of between 3 and 7 dB, according to the specific path of the light beam within the switching matrix). In order to reduce the optical budget in the switching matrix and increase the number of input/output openings, it is therefore necessary to reduce the insertion losses.
In particular, the insertion losses are linked to the geometry of the cavities in which the bubbles form and, specifically, are correlated to the size of the cavities themselves, as explained in what follows.
In this connection, reference may be made to FIG. 1 illustrating a top view of a portion of an optical switching device 1 of the type considered, including two waveguides 2a, 2b, which are perpendicular to one another and intersect at a chamber 3, inside which there is present a liquid having, in normal conditions, optical properties similar to those of the waveguides 2a, 2b. 
As may be better seen in the cross section of FIG. 2, the optical switching device 1 is formed by an optical switching chip 4, a switching control chip 5, and a reservoir chip 6, which are bonded together in such a way as to form a sandwich.
The optical switching chip 4, defining a planar lightwave circuit (PLC), comprises a substrate 20 of semiconductor material (silicon) and an optical layer 21, made of glass (quartz), which houses the waveguides 2a, 2b and is deposited on top of the substrate 20. The waveguides 2a, 2b are preferably formed by (in a known way, which is not illustrated) a core of glass doped in such a way as to have a first refractive index, surrounded by a cladding doped in such a way as to have a second refractive index, so as to optimize transmission of the light beam and enable transmission of secondary wave modes or eliminate them according to the requisites imposed.
FIG. 2 illustrates a single waveguide 2a and a single chamber 3, which is formed by a first cavity 8 and a second cavity 12 facing one another. The first cavity 8 is formed in the optical layer 21 and faces a bottom surface 4a of the optical switching chip 4. The optical-switching chip 4 is fixed, on its bottom surface 4a, to a top surface 5a of the switching control chip 5 by means of a hermetic O-ring 9, which delimits, together with the optical switching chip 4 and the switching control chip 5, a gap 10.
The second cavity 12 is formed in the switching control chip 5 and faces the top surface 5a of the latter. Beneath the second cavity 12 there is formed a resistor 11 connected to electronic control components (not illustrated and integrated into the switching control chip 5) which control passage of electric current through the resistor 11 according to the switching scheme desired. A through channel 13 extends inside the switching control chip 5 from the gap 10 as far as a bottom surface 5b of the switching control chip 5. The switching control chip 5 is fixed, on its bottom surface 5b, to the reservoir chip 6, by means of a hermetic bonding layer 15.
The reservoir chip 6, which is preferably also made of silicon and which houses a reservoir 16 facing the bottom surface 5b of the switching control chip 5, is here connected to the through channel 13 and terminates laterally for connection with a filling tube 17 on which a device 18 for controlling the pressure is arranged.
The filling tube 17, the reservoir 16, the through channel 13, the second cavity 12, the gap 10, and the cavity 8 are filled with a liquid having a refractive index equal to that of the waveguides 2a, 2b. In FIG. 2, a bubble 19 is moreover present in the chamber 3.
Given that the optical layer 21 is made of quartz and the switching control chip 5 is made of silicon, soldering thereof is critical, and stresses may occur in the structures such as to cause bowing of the optical switching device 1.
In addition, the O-ring 9 has a thickness of a few micron, which determines the height of the gap 10 and increases the distance between the resistor 11 and the first cavity 8. The presence of the gap 10 is disadvantageous and can cause problems of stability of the bubble 19 over time or a reduction in the transmission characteristics of the switching circuit, depending on the geometry of the chamber 3.
In particular, if the cavities 8, 12 are very wide, in such a way as to contain the bubble 19 completely in the lateral direction without the bubble extending longitudinally inside the gap 10 (as shown in FIG. 3), there is a high stability of the bubble. However, in this case the geometry of the bubble 19 causes high propagation losses, and the optical circuit has, as a whole, relatively high insertion losses.
If, instead, the size of the cavity 8 is small, such that the bubble 19 is not contained completely in the lateral direction and extends in width also inside the gap 10 (as shown in FIG. 4), there is a reduction in the propagation losses of the optical circuit, but the bubble becomes unstable over time, and its conformation depends upon the thermal conditions existing in the area surrounding the chamber 3.
In order to improve the stability of the bubbles, the ideal solution would be to concentrate as much as possible the distribution of the heat generated by the resistor 11 in such a way as to have a restricted heat spot, only inside the chamber 3, maintaining the size of the chamber 3 as small as possible in order to reduce the total insertion loss of the matrix. At present, however, this is not possible in so far as the presence of liquid in the gap 10 causes heating of a much wider area, even when the resistor 11 extends only underneath the second cavity 12.
Embodiments of the present invention provide an optical device that overcomes the drawbacks of the known solutions.
Aspects of the present invention involve an optical device having a first chip including an optical layer of dielectric material housing an optical circuit. Further aspects involve a second chip including a body of semiconductor material housing integrated electronic components and bonded to said first chip. In accordance with other aspects of the invention, the first chip and the second chip respectively have a first cavity and a second cavity facing one another to form a chamber filled with a liquid. The second chip includes a lateral delimiting region, made of rigid material, arranged between said body of semiconductor material and said first chip and delimiting laterally said chamber.